1. Field of the Invention
This invention relates to amplifiers and, more specifically, to amplifiers which provide low-distortion operation while requiring relatively little headroom.
2. Description of the Related Art
Bipolar transistor amplifiers are employed in a wide variety of applications. They are commonly used, for example, as intermediate frequency (IF) amplifiers within radio frequency (RF) receivers.
The conventional bipolar transistor amplifier of FIG. 1 includes NPN transistors Q1 and Q2 connected as a Darlington pair to increase the current gain through the stage and to increase the input impedance. A feedback resistor Rf is connected in series with a base resistor Rb to form a divider between a positive supply voltage V+ and a negative supply voltage V- and to thereby provide bias current to the Darlington pair Q1, Q2. A collector resistor Rc is connected between the positive supply terminal V+ and the Darlington pair's collectors. The resistor Rc minimizes the Darlington's bias current variation over temperature. Resistor Rcomp and capacitor Ccomp, which together, are in parallel with Rf, provide frequency compensation for the amplifier. A peaking inductor L1, typically on the order of a few microhenrys is connected in series between the collector resistor Rc and the Darlington's collectors in order to cancel the effects of stray capacitances and to extend the gain of the amplifier above the normal rolloff frequency. High-frequency techniques such as this are known in the art and are discussed in Paul Horowitz, Winfield Hill, The Art of Electronics, Cambridge University Press, New York, 1989, page 869.
Respective degeneration resistors Re1 and Re2 are connected to the emitters of transistors Q1 and Q2. Since signal distortion is inversely proportional to the collector emitter voltage Vce2 of the output transistor Q2,the output transistor Q2 is generally biased at a very high current. Consequently, the degeneration resistor Re2 is typically a very low-valued resistor, i.e. on the order of a few ohms.
Although a conventional amplifier such as this operates satisfactorily for many applications, it is not particularly suited for low-power and/or low supply-voltage applications. One deficiency of the circuit involves the collector resistor Rc, which increases the amplifier's headroom requirement. In some applications, particularly low supply-voltage applications, this increased headroom requirement can be difficult to satisfy. That is, it is generally good design practice to use a single, standard, power supply voltage for a given electronics system. For example, many electronics systems now employ a three-volt power supply. But the voltage drop across the dropping resistor Rc, in combination with the need for high output current (to maximize Vce and thereby minimize distortion), may force a designer to add a higher-voltage power supply to the system in order to use the amplifier of FIG. 1. Another problem is that the resistor Rc dissipates a significant amount of power. This, in itself, is a significant disadvantage. In addition, the increased power dissipation can raise the junction temperature of the Darlington pair, thereby accelerating electro-migration and reducing the expected life-time of the amplifier.
Additionally, the degeneration resistor Re2 must be a low-value resistor, on the order of a few ohms, in order to meet the amplifier's headroom requirement. Due to the resistor's low value, a small variation in its resistance, due for example to manufacturing tolerances, can lead to a significant change in the amplifier's bias current.